In the field of wireless communication, recently, the technique of relaying a radio signal has attracted attention.
The relaying technique is expected to exert effects of expansion of the communication area and increase of the communication capacity. A relay station (RS: Relay Station) is installed to cover a place where a radio wave transmitted from a base station (BS: Base Station) does not reach. Then, the relay station relays the radio signal between the BS and a mobile station (MS: Mobile Station), with the result that the base station can communicate with the mobile station via the relay station. Therefore, it is possible to expand the communication area. The transmission power of the mobile station largely affects the power consumption of a battery. When the relaying technique is applied to an uplink which is a channel from the mobile station to the base station, an effect that the transmission power of the mobile station is suppressed to a low level is remarkably obtained. In the relaying technique, when the communication area covered by the RS is set narrow, a frequency can be repeatedly used. Consequently, there is an effect that the frequency use efficiency of the entire system is improved and the communication capacity can be increased.
Hereafter, a base station is referred to as a BS, a relay station (relay station apparatus) is referred to as an RS, and a mobile station is referred to as an MS. In the case where a plurality of stations exist, they are expressed as BSi (i=1, 2, 3 . . . . , n−1, n; n is a natural number), RSi (i=1, 2, 3 . . . . , n−1, n; n is a natural number), and MSi (i=1, 2, 3 . . . . , n−1, n; n is a natural number), respectively.
As a relaying method in an RS, there are various methods. When attention is focused on the signal processing in the RS, for example, there are a method (decode and forward relay) in which a signal received by the RS is demodulated and decoded to reproduce and transmit (relay) transmission data, and a method (amplify and forward relay) in which power amplification is performed on an RF signal received by the RS and the power-amplified RF signal is transmitted (relayed). When attention is focused on the number of RSs which perform relaying, there are a method (multi-hop) in which relaying is sequentially performed by using a plurality of RSs, and a method (single hop) in which relaying is performed by using a single RS.
Referring to FIG. 17, a multi-hop in a channel (uplink) from an MS to a BS will be described. FIG. 17 shows a multi-hop in an uplink. In FIG. 17, in the uplink, RS1, RS2, RS3, RSn−1, and RSn relay a signal transmitted from an MS, to a BS. The arrows in the figure indicate relay paths from the MS to the BS.
As indicated by the arrows (thick channels) shown in FIG. 17, RS3 first relays the signal transmitted from the MS. Next, the relay station RSn−1 transmits (relays) the signal relayed by RS3, to the BS. As a method of controlling a relay path from the MS to the BS, for example, there are a method in which RSs control the relay paths in an autonomous decentralized manner, respectively, and a method in which the relay paths are controlled in a centralized manner by using a path controlling apparatus.
In the multi-hop environment which has been described with reference to FIG. 17, the technique called flooding has been studied.
Flooding in the multi-hop environment will be described with reference to FIG. 18. FIG. 18 is a diagram showing flooding in an uplink. In FIG. 18, an environment where a signal transmitted from the MS cannot directly reach the BS, and signal relaying by the RS is necessary is set. Furthermore, RS1, RS2, RS3, and RSn transmit (relay) the signal transmitted from the MS. Moreover, it is assumed that the relay paths indicated by the arrows in the figure from the MS to the BS have not been determined.
Referring to FIG. 18, after the MS transmits the signal, the RSs receive the transmitted signal of the MS and perform a receiving process because the relay path of the transmitted signal has not been determined. Next, each of the RSs determines whether it has received the transmitted signal of the MS or not. As a method in which each RS determines whether it has received the transmitted signal of the MS or not, for example, there are a method in which it is determined depending on the reception level whether the reception has been performed or not, and a method in which, in the case of decode and forward relay, determination is performed by using a function of error detection.
Then, all of RSs which determine that the reception has been performed perform relaying. In the figure, a circle and “RECEPTION OK” are shown at the tip end of an arrow indicating the signal of the determination that the corresponding RS has performed the reception. Moreover, a cross and “RECEPTION NG” are shown at the tip end of an arrow indicating the signal of the determination that the corresponding RS has failed the reception.
In FIG. 18, as a result of the receiving process which has been performed in each of RS1, RS2, and RS3 on the signal transmitted by the MS, RS1 and RS2 determine that the reception has been performed, and RS3 determines that reception cannot be performed. Therefore, RS1 and RS2 relay the transmitted signal of the MS.
In FIG. 18, next, it is determined that the signal relayed by RS2 has been transmitted to RSn and RSn has received the signal. Then, RSn relays the signal transmitted from RS2, to the BS. In this way, the transmitted signal of the MS reaches the BS while repeating the relaying in the RSs.
In the flooding in the multi-hop environment shown in FIG. 18, for example, a method of preventing a signal that is once received by an RS from being twice relayed in order not to cause a signal loop, and a method of avoiding signal collision in order to prevent a plurality of RSs to perform transmission at the same timing are studied. Moreover, application to broadcast transmission in the multi-hop environment, a sensor network, and the like is widely studied.
Similarly, application of flooding to point-to-point communication is also studied.
Non-patent Literature 1 discloses a relay method in which flooding is applied to point-to-point communication. In the method, specifically, among a plurality of RSs which determine that a signal transmitted from an MS has been received, the RS having the highest channel quality from the RS to a BS relays the signal transmitted from the MS.
Here, the relay method disclosed in Non-patent Literature 1 above will be described with reference to FIGS. 19 and 20. FIG. 19 is a diagram showing the manner of relaying in an uplink. FIG. 20 is a diagram showing a relaying operation shown in FIG. 19.
FIGS. 19 and 20 assume an environment where the signal transmitted from the MS cannot directly reach the BS, and therefore signal relaying by RSs is necessary. It is assumed that the relay path from the MS to the BS has not been determined. In FIGS. 19 and 20, moreover, it is assumed that the downlink signal from the BS can be received by the MS and all the RSs. This is because limitation of the transmission power in the BS is smaller than that in the MS and hence the downlink signal can be transmitted with a large power. In FIGS. 19 and 20, furthermore, it is assumed that the channel quality from an RS to the BS is higher in the sequence of RS3, RS2, and RS1. The channel quality from an RS to the BS is estimated by each RS from the downlink signal from the BS. In FIGS. 19 and 20, moreover, it is assumed that the channel quality from an RS to the BS is higher in the sequence of RS3, RS2, and RS1. This is because it is considered that the channel quality from an RS to the BS is higher as the distance between the RS and the BS is shorter. The arrows in FIGS. 19 and 20 indicate the transmitted signal of the MS. Numbers (1) to (4) are affixed to the arrows in accordance with the relay destinations, respectively.
With reference to FIGS. 19 and 20, first, the MS transmits the signal, and thereafter the RSs perform the process of receiving the signal transmitted from the MS. Then, each of the RSs determines whether it has received the signal transmitted from the MS or not. As the method by which the RS determines that the reception has been performed, there are a method in which it is determined depending on the reception level whether the reception has been performed or not, and a method in which, in the case of decode and forward relay, determination is performed by using a function of error detection.
In FIGS. 19 and 20, next, among RS1 and RS2 which determine that reception has been performed, the RS2 having the highest channel quality from the RS to the BS is selected as an RS which is to relay the signal transmitted from the MS, to the BS. Then, RS2 transmits the signal transmitted from the MS, to the BS.
As described above, an RS which is estimated that the channel quality from the RS to the BS is highest is selected as an RS which is to relay the signal transmitted from the MS, to the BS, and therefore an RS which is closer to the BS is selected. In the multi-hop environment, therefore, the number of relays can be reduced. In the single-hop environment, the channel quality from an RS to the BS is high, and hence the reception performance in the BS becomes excellent. In the single-hop environment, in the case where adaptive modulation is applied in the signal transmitted from an RS, the frequency efficiency can be improved.
In the flooding, in order not to cause a signal loop, as described above, the configuration where an RS does not relay twice a signal that is once received is studied. In order to prevent an RS from relaying twice a signal that is once received, the RS must notify the other RSs that relaying is performed by the RS oneself, by any method. With respect to the channel quality from the RS to the BS, for example, a waiting time corresponding to the relay priority is previously set in the time from the timing when the RS receives the signal to that when the RS relays it.
With reference to FIG. 21, therefore, a method of notifying RSs other than the relaying RS that the relaying RS relays a signal will be described. FIG. 21 is a diagram showing relationships between the waiting time (ordinate) corresponding to the relay priority and the channel quality (abscissa) from an RS to the BS.
In the method, as shown in FIG. 21, in the time from the timing when an RS receives the signal to the timing when the RS relays it, an RS having a higher relay priority earlier starts relaying with respect to the waiting time corresponding to the relay priority. In each of the RSs, during the waiting time of the RS oneself, in the case where a relayed signal from another RS is not detected, the RS oneself performs relaying.
In the case of the past example shown in FIGS. 19 and 20, the channel quality from an RS to the BS is higher in the sequence of RS2 and RS1 except RS3 which has failed to receive the transmitted signal from the MS. With reference to FIG. 21, it is seen that the waiting time of RS2 in which the channel quality from the RS to the BS is higher than RS1 is shorter than that of RS1. Therefore, RS2 relays the transmitted signal from the MS more preferentially than RS1. Since the transmitted signal from the MS is already relayed from RS2, RS1 does not relay the transmitted signal from the MS.